TOKYO, Japan (AP) -- The Japanese government recently looked into the possibility of developing a nuclear warhead, a news report said Monday, citing an internal government document.

However the government's top spokesman Yasuhisa Shiozaki denied such a document existed.

The Japanese daily Sankei reported that experts at several government organizations concluded it would take at least three to five years to make a prototype weapon.

The experts also estimated that the project would cost about $1.68 billion to $2.52 billion and require the efforts of several hundred engineers, according to Sankei.

The experts did not say whether Japan should develop nuclear arms, the newspaper reported, only what such a project would require. The newspaper published a summary of the document, dated Sept 20 and titled "On the Possibility of Developing Nuclear Weapons Domestically."

"The government is not aware of such a document," Shiozaki told reporters at a regular news conference.

As the only country ever attacked with atomic weapons, Japan for decades has adhered to a strict policy of not possessing or developing nuclear weapons, and not allowing their introduction onto Japanese territory.

This stance, however, has become a subject for discussion since North Korea conducted its first nuclear test on Oct. 9, causing deep concern in Japan. Just months prior to North Korea's nuclear test, it test-fired several ballistic missiles capable of hitting Japan.

Several politicians have suggested Japan should at least debate starting a nuclear weapons program following the North Korean test.

The government of Prime Minister Shinzo Abe has said the country's pacifist Constitution does not ban it from possessing nuclear weapons for self-defense. But the government stressed that Japan would stick to its policy of forbidding nuclear weapons on Japanese soil.

Japan's huge plutonium stockpile from its nuclear power stations is a major international concern, partly because that stockpile could be a target of terror attacks or be used to build nuclear arms.

Officials at the Defense Agency could not immediately comment on the report early Monday.